The development of surgical techniques have made great progress over the years. For instance, patients who need to be operated in the brain, can nowadays undergo non-invasive surgery with very little trauma to the patient.
Leksell Gamma Knife® provides such surgery by means of gamma radiation. The radiation is emitted from fixed radioactive sources and are focused by means of collimators, i.e. passages or channels for obtaining a beam of limited cross section, toward a defined target. Each of the sources provide only a small dose to intervening tissue, with the resulting maximum radiation dose available only at the common focus where the radiation beams intersect. The target volume is determined, e.g. depending on the size of tumor to be radiated, by selecting different sizes of collimators. A type of collimator body called a “helmet”, shaped like a hemisphere, is provided with collimators of one defined size which at the target area or a site of focus provides a certain radiation beam diameter. If another beam diameter is desired, the helmet must be changed to another helmet having a suitable size of collimators.
Even though the radiation therapy can be successfully carried out with the above described equipment, it may be desirable to reduce the time which lapses during the changing of helmets.
U.S. Pat. No. 5,757,886 discloses a medical radiating unit, in which a collimator carrier carries several sets of collimators of different aperture diameters distributed in correspondence with the distribution of radioactive sources in a source carrier. The collimator carrier and the source carrier may be rotated relative to each other, thereby enabling a change from one set of collimators to another set of collimators. This eliminates the need to use several helmets in order to be able to change collimator sizes. However, it is still quite limited as regards possibilities to choose different spatial dose distributions surrounding the focus. Another example is a whole-body radiotherapy device as disclosed in EP 1057499, which is also quite limited as regards variation of spatial dose distribution.